Our goal is to learn how the self-renewing ability of stem cells is achieved?a question central to the understanding of tissue development, maintenance, and repair, with profound implications in regenerative medicine and cancer treatment. Our strategy is to use Drosophila as a model to address this question, and to extend what we learn from Drosophila to mammalian and clinical settings. We previously identified germline stem cells (GSCs) in Drosophila and revealed the self-renewing pattern of their divisions. We then discovered key genes that define inter- and intracellular mechanisms of stem cell division, and demonstrated the essential role of intercellular signaling in GSC maintenance. This has led to the development of the current stem cell niche theory. Moreover, our studies of the piwi (a.k.a., argonaute) family genes, the only known family of genes with stem cell function highly conserved in both animal and plant kingdoms, start to link epigenetic regulation to stem cell function. Our focus will be on how epigenetic mechanisms dictate stem cell fate. We will investigate what regulates changes of chromatin organization and activity to generate the unique gene expression profile of the genome that defines stem cell fate. Specifically, we are in a unique position to study epigenetic program-ing mediated by PIWI family proteins and small RNAs?an exciting new frontier of stem cell research. Our working hypothesis is that PIWI controls stem cell fate by regulating the transcriptional activity of the chromatin via heterochromatin protein 1 (HP1) and small RNA-mediated processes. This regulation can occur in stem cells or niche cells. To test and to further develop this hypothesis, we propose to: 1. Test the fundamental hypothesis that epigenetic programming determines stem cell fate. 2. Define and characterize the functional domains of PIWI. 3. Identify PlWI-interacting proteins and characterize their potential epigenetic function in the stem/niche cell genomes. 4. Identify the small RNA targets of PIWI and characterize their function in the epigenetic regulation of stem and niche cell genomes.